Platelet transfusions are a common therapeutic maneuver for a variety of diseases that result in low platelet counts, which can lead to bleeding complications. A substantial number of transfused patients require platelets due to non-malignant hematopoietic disorders (either bone marrow failure syndromes or genetic defects in hematopoiesis), the only cure for which is bone marrow transplants (BMT). It has been observed that transfused patients have higher rates of BMT rejection, thus limiting the feasibility of utilizing BMT as a cure. A number of explanations for the increased rejection rates have been suggested, including immunological effects. We have recently reported in an animal model, that platelet transfusions in of themselves immunize against transplantation antigens that can then cause subsequent BMT rejection. Since BMT in this setting is MHC matched, these antigens are minor histocompatibility antigens (mHAs). Previously, it has been assumed that contaminating leukocytes in blood transfusions were the main source of immunization to minor antigens. However, the implementation of stringently leukoreduced blood products (fewer than 1x106 total leukocytes per unit of blood) has not decreased the rate of BMT rejection in chronically transfused patients. Based upon these findings, we hypothesized that non-leukocyte components are responsible for immunization against mHAs, and in this case, the platelets themselves. To address this hypothesis, and to closely model human transfusion, we have developed procedures to isolate and filter leukoreduce murine platelets using the same techniques and filters as are used in humans. Our data demonstrate that stringently filter leukoreduced platelets still induce BMT rejection. In support of this concept, we present data in this application to indicate that mHAs on transfused platelets are crosspresented into the MHC class I pathway of recipient antigen presenting cells (APCs), resulting in activation and expansion of recipient CD8+ T cells specific for the mHAs. Avoiding immunization by avoiding transfusion is not feasible, as the transfused platelets fulfill a therapeutic necessity; thus, generating strategies to circumvent the immune barriers will be required. The rational development of new approaches to avoid immunization requires a more detailed mechanistic understanding of the immunization and subsequent transplant rejection that are caused by platelet transfusion. We propose to study the mechanisms of platelet transfusion induced BMT rejection through the following specific aims. Specific Aim 1: Elucidate the immune mechanisms of PLT transfusion-induced BMT rejection. Specific Aim 2: Analysis of CD4+ T and CD8+ T cell immunization by mHAs carried by transfused PLTs. Specific Aim 3: Differential immunogenicity of distinct cell subsets in transfused blood. Together, the proposed aims will provide a mechanistic elucidation of how platelet transfusion induces subsequent BMT rejection. These studies have the potential to directly benefit patient populations who require platelet transfusions and subsequent BMT.